Cellulose is a polymer of D-glucose and is a structural component of plant cell walls. Cellulose is especially abundant in tree trunks from which it is extracted, converted into pulp, and thereafter utilized to manufacture a variety of products. Rayon is the name given to a fibrous form of regenerated cellulose that is extensively used in the textile industry to manufacture articles of clothing. For over a century strong fibers of rayon have been produced by the viscose and cuprammonium processes. The latter process was first patented in 1890 and the viscose process two years later. In the viscose process cellulose is first steeped in a mercerizing strength caustic soda solution to form an alkali cellulose. This is reacted with carbon disulfide to form cellulose xanthate which is then dissolved in dilute caustic soda solution. After filtration and deaeration the xanthate solution is extruded from submerged spinnerets into a regenerating bath of sulfuric acid, sodium sulfate, zinc sulfate, and glucose to form continuous filaments. The resulting so-called viscose rayon is presently used in textiles and was formerly widely used for reinforcing rubber articles such as tires and drive belts.
Cellulose is also soluble in a solution of ammoniacal copper oxide. This property forms the basis for production of cuprammonium rayon. The cellulose solution is forced through submerged spinnerets into a solution of 5% caustic soda or dilute sulfuric acid to form the fibers, which are then decoppered and washed. Cuprammonium rayon is available in fibers of very low deniers and is used almost exclusively in textiles.
The foregoing processes for preparing rayon both require that the cellulose be chemically derivatized or complexed in order to render it soluble and therefore capable of being spun into fibers. In the viscose process, the cellulose is derivatized, while in the cuprammonium rayon process, the cellulose is complexed. In either process, the derivatized or complexed cellulose must be regenerated and the reagents that were used to solubilize it must be removed. The derivatization and regeneration steps in the production of rayon significantly add to the cost of this form of cellulose fiber. Consequently, in recent years attempts have been made to identify solvents that are capable of dissolving underivatized cellulose to form a dope of underivatized cellulose from which fibers can be spun.
One class of organic solvents useful for dissolving cellulose are the amine-N oxides, in particular the tertiary amine-N oxides. For example, Graenacher, in U.S. Pat. No. 2,179,181, discloses a group of amine oxide materials suitable as solvents. Johnson, in U.S. Pat. No. 3,447,939, describes the use of anhydrous N-methylmorpholine-N-oxide (NMMO) and other amine N-oxides as solvents for cellulose and many other natural and synthetic polymers. Franks et al., in U.S. Pat. Nos. 4,145,532 and 4,196,282, deal with the difficulties of dissolving cellulose in amine oxide solvents and of achieving higher concentrations of cellulose.
Lyocell is an accepted generic term for a fiber composed of cellulose precipitated from an organic solution in which no substitution of hydroxyl groups takes place and no chemical intermediates are formed. Several manufacturers presently produce lyocell fibers, principally for use in the textile industry. For example, Acordis, Ltd. presently manufactures and sells a lyocell fiber called TENCEL.RTM. fiber.
Currently available lyocell fibers suffer from one or more disadvantages. One disadvantage of some lyocell fibers made presently is a function of their geometry which tends to be quite uniform, generally circular or oval in cross section and lacking crimp as spun. In addition, many current lyocell fibers have relatively smooth, glossy surfaces. These characteristics make such fibers less than ideal as staple fibers in woven articles since it is difficult to achieve uniform separation in the carding process and can result in non-uniform blending and uneven yarn.
In addition, fibers having a continuously uniform cross section and glossy surface produce yarns tending to have an unnatural, "plastic" appearance. In part to correct the problems associated with straight fibers, man-made staple fibers are almost always crimped in a secondary process prior to being chopped to length. Examples of crimping can be seen in U.S. Pat. Nos. 5,591,388 or 5,601,765 to Sellars et al. where a fiber tow is compressed in a stuffer box and heated with dry steam. Inclusion of a crimping step increases the cost of producing lyocell fibers.
Another widely-recognized problem associated with prior art lyocell fibers is fibrillation of the fibers under conditions of wet abrasion, such as might result during laundering. Fibrillation is defined as the splitting of the surface portion of a single fiber into smaller microfibers or fibrils. The splitting occurs as a result of wet abrasion caused by attrition of fiber against fiber or by rubbing fibers against a hard surface. Depending on the conditions of abrasion, most or many of the microfibers or fibrils will remain attached at one end to the mother fiber. The microfibers or fibrils are so fine that they become almost transparent, giving a white, frosty appearance to a finished fabric. In cases of more extreme fibrillation, the microfibers or fibrils become entangled, giving the appearance and feel of pilling, i.e., entanglement of fibrils into small, relatively dense balls.
Fibrillation of lyocell fibers is believed to be caused by the high degree of molecular orientation and apparent poor lateral cohesion of microfibers or fibrils within the fibers. There is extensive technical and patent literature discussing the problem and proposed solutions. As examples, reference can be made to papers by Mortimer, S. A. and A. A. Peguy, Journal of Applied Polymer Science, 60:305-316 (1996) and Nicholai, M., A. Nechwatal, and K. P. Mieck, Textile Research Journal 66(9):575-580 (1996). The first authors attempt to deal with the problem by modifying the temperature, relative humidity, gap length, and residence time in the air gap zone between extrusion and dissolution. Nicholai et al. suggest crosslinking the fiber but note that ". at the moment, technical implementation [of the various proposals] does not seem to be likely". A sampling of related United States patents includes those to Taylor, U.S. Pat. Nos. 5,403,530, 5,520,869, 5,580,354, and 5,580,356; Urben, U.S. Pat. No. 5,562,739; and Weigel et al. U.S. Pat. No. 5,618,483. These patents in part relate to treatment of the fibers with reactive materials to induce surface modification or crosslinking. Enzymatic treatment of yarns or fabrics is currently the preferred way of reducing problems caused by fibrillation; however, all of the treatments noted have disadvantages, including increased production costs.
Additionally, it is believed that currently available lyocell fibers are produced from high quality wood pulps that have been extensively processed to remove non-cellulose components, especially hemicellulose. These highly processed pulps are referred to as dissolving grade or high alpha (or high .alpha.) pulps, where the term alpha (or .alpha.) refers to the percentage of cellulose. Thus, a high alpha pulp contains a high percentage of cellulose, and a correspondingly low percentage of other components, especially hemicellulose. The processing required to generate a high alpha pulp significantly adds to the cost of lyocell fibers and products manufactured therefrom.
For example, in the Kraft process a mixture of sodium sulphide and sodium hydroxide is used to pulp the wood. Since conventional Kraft processes stabilize residual hemicelluloses against further alkaline attack, it is not possible to obtain acceptable quality dissolving pulps, i.e., high alpha pulps, through subsequent treatment in the bleach plant. In order to prepare dissolving type pulps by the Kraft process, it is necessary to give the chips an acidic pretreatment before the alkaline pulping stage. A significant amount of material, on the order of 10% of the original wood substance, is solubilized in this acid phase pretreatment. Under the prehydrolysis conditions, the cellulose is largely resistant to attack, but the residual hemicelluloses are degraded to a much shorter chain length and can therefore be removed to a large extent in the subsequent Kraft cook by a variety of hemicellulose hydrolysis reactions or by dissolution. Primary delignification also occurs during the Kraft cook.
The prehydrolysis stage normally involves treatment of wood at elevated temperature (150-180.degree. C.) with dilute mineral acid (sulfuric or aqueous sulfur dioxide) or with water alone requiring times up to 2 hours at the lower temperature. In the latter case, liberated acetic acid from certain of the naturally occurring polysaccharides (predominantly the mannans in softwoods and the xylan in hardwoods) lowers the pH to a range of 3 to 4.
While the prehydrolysis can be carried out in a continuous digester, typically the prehydrolysis is carried out in a batch digester. As pulp mills become larger and the demand for dissolving grade pulp increases, more batch digesters will be needed to provide prehydrolyzed wood. The capital cost of installing such digesters and the costs of operating them will contribute to the cost of dissolving grade pulps. Further, prehydrolysis results in the removal of a large amount of wood matter and so pulping processes that incorporate a prehydrolysis step are low yield processes.
Moreover, a relatively low copper number is a desirable property of a pulp that is to be used to make lyocell fibers because it is generally believed that a high copper number causes cellulose degradation during and after dissolution in an amine oxide solvent. The copper number is an empirical test used to measure the reducing value of cellulose. Further, a low transition metal content is a desirable property of a pulp that is to be used to make lyocell fibers because, for example, transition metals accelerate the degradation of cellulose and NMMO in the lyocell process.
Thus, there is a need for relatively inexpensive, low alpha pulps that can be used to make lyocell fibers, for a process for making the foregoing low alpha pulps, and for lyocell fibers from the foregoing low alpha pulp. Preferably the desired low alpha pulps will have a low copper number, a low lignin content and a low transition metal content. Preferably it will be possible to use the foregoing low alpha pulps to make lyocell fibers having a decreased tendency toward fibrillation and a more natural appearance compared to presently available lyocell fibers.